1. Field of the Invention
This invention relates generally to receivers and, more particularly, to receivers that include bit synchronizers.
2. Discussion of the Related Art
In some circumstances, transmitters may have a bit rate that is significantly less than the sample rate of a receiver. For example, several transmitter signals (such as UHF/VHF and other functions) that are transmitted to an Integrated Communications, Navigation and Identification Avionics (ICNIA) system of a Comanche helicopter have a 16 Kbit/second data mode. The UHF/VHF receiver that is used in the Comanche ICNIA system produces a baseband digital signal at a nominal 64 Ksample/second rate. In other words, there are normally 4 samples per baseband symbol. For AM signals, the received signal is positive-valued and corresponds to the envelope detected for on-off keying (OOK) for these functions. F or FM signals, the signal is bipolar or antipodal, corresponding to frequency shift keying (FSK). Currently, there is no device that is associated with a receiver that can alter the timing of the sampling clock of the receiver. Furthermore, a numerically-controlled oscillator and feedback loop are not available.
To successfully receive the data, the receiver must determine the unknown starting and ending samples of each data symbol—a process that is known as bit synchronization. Bit synchronization is obscured by received noise, signal fading, relative time drift and time jitter between the received symbol sequence and the sampling clock of the receiver. Control of the sampling clock of the receiver is not available to the bit synchronizer. To be successful, the bit synchronizer must operate strictly on the received data stream. The bit synchronizer must also determine the symbol timing, even when the transmit clock symbol rate and the receiver sampling rate are offset and have a relative drift.